Image-forming device

ABSTRACT

Flanges are pressed into opposite ends of a photoreceptor drum. A noncontact charging roller is arranged so as to face, but have no direct contact with, the photoreceptor drum. On both end portions of the noncontact charging roller, spacers are provided for maintaing a gap between the photoreceptor drum and the noncontact charging roller. The spacers are of tape form and wound around the noncontact charging roller. Winding positions of the spacers are distant by more than an effective projection length of each of the flanges from respective opposite ends of the charging roller.

TECHNICAL FIELD

This invention relates to electrophotographic image forming apparatususing a noncontact charging method.

BACKGROUND ART

In conventional image forming apparatus such as electrophotographiccopying machines, a surface of a photoreceptor (a charged member) ispositively or negatively charged uniformly by a corona discharge device.In a subsequent exposure process, certain points of the surface areselectively discharged to form an electrostatic latent image. Then, adeveloper supplying device with a predetermined amount of developingbias applied supplies developer to the surface of the photoreceptor, sothat the latent image is visualized, i.e., developed.

Some image forming apparatus using the corona discharge method areprovided with a combined developing/cleaning device. Such image formingapparatus uses a toner scattering process, instead of a dedicatedcleaning device. In the toner scattering process, an electricallyconductive brush scatters residual toner particles remaining on thephotoreceptor after a preceding transfer process. Also, such apparatusadopts a developing process using magnetic toner. See Japanese examinedPatent Application No. H06-50416, p.3, left column, lines 4 to 7.

The combined developing/cleaning device allows for downsizing of suchapparatus. However, the corona discharge device provided in suchapparatus is easily affected by environmental factors such as humidityor dust. Also, the corona discharge process involves ozone emissionswhich have an unpleasant odor and possible harmful effects on humanhealth.

One solution to the foregoing problems is a contact charging method inwhich a surface of a charged member (photoreceptor drum) is charged bydirect contact with a conductive member (charging roller) to which adirect-current voltage with an alternating-current voltage superposed isapplied.

The contact charging method, however, causes problems as describedbelow. In an image forming apparatus using the contact charging method,a conductive member (charging roller) becomes in direct contact with asurface of a charged member (photoreceptor drum). Accordingly, whenthere are relatively hard particles, such as toner carriers, on thesurfaces of the charged member and the conductive member, the particlesscratch the surfaces when the surfaces become in contact with eachother. Also, foreign particles which adhere to a portion of the surfaceof the conductive material (charging roller) cause a correspondingportion of the surface of the charged member (photoreceptor drum) to benon-uniformly charged.

To solve the foregoing problems of the contact charging method as wellas to achieve the greatest advantage thereof, i.e., no ozone emission,there has been proposed a noncontact charging method in which a chargingmember is positioned in proximity to (thus, out of contact with) aphotoreceptor. See FIG. 1 of JP-H05-307279-A, or FIG. 1 ofJP-H07-301973-A.

Application of the noncontact charging method to an image formingapparatus provided with a two-component developing device has also beenproposed. See paragraph [0019], and FIG. 1, of JP-2001-188403-A. In theapparatus, a narrowest gap between a discharging surface of a chargingmember and a photoreceptor is rendered larger than diameter of a tonercarrier particle. This prevents a toner carrier, or toner carried on thetoner carrier, from getting stuck in the gap. Thus, the toner carrier isprevented from scratching or contaminating the surfaces of thephotoreceptor and the charging member.

However, the apparatus as disclosed by JP-2001-188403-A does not have acombined developing/cleaning device such as disclosed by Japaneseexamined Patent Application No. H06-50416. Consequently, the apparatustends to grow in size and to require a high supply voltage. Also, sincethe narrowest gap between the surfaces of the charging member and thephotoreceptor is rendered larger than the diameter of the toner carrierparticle, an extra amount of voltage is required for charging thephotoreceptor.

Further, if the gap is rendered smaller than the diameter of the tonercarrier particle to solve the problem, a voltage applied to the chargingroller is reduced. However, fluctuations in gap width may have greatereffects, and therefore the gap width has to be maintained with highprecision. Furthermore, a cleaning process is required to be performedon an upstream side of the photoreceptor and the charging roller inorder to prevent the photoreceptor or the charging roller from beingscratched or contaminated. The cleaning process potentially causes anincrease in load torque, or abrasion of, and scratches on, the surfaceof the photoreceptor.

A feature of the invention is to offer an image forming apparatus usingthe noncontact charging method, capable of precisely adjusting a gapbetween a non-contact charging roller and a photoreceptor, so that thephotoreceptor is prevented from being nonuniformly charged because ofabnormal discharge or insufficient charging and therefore high qualityimage is ensured.

DISCLOSURE OF THE INVENTION

An image forming apparatus according to the invention includes aphotoreceptor drum that has flanges pressed into opposite ends thereof,a noncontact charging roller that is arranged so as to face thephotoreceptor drum but to have no direct contact with the photoreceptordrum, and spacers for maintaining a gap between the photoreceptor drumand the noncontact charging roller. The spacers are wound aroundopposite end portions of the noncontact charging roller. Windingpositions of the spacers are distant by more than an effectiveprojection length of each of the flanges from the respective oppositeends of the charging roller.

An outside diameter of a photoreceptor body increases across portions ofthe photoreceptor body into which the flanges are pressed, i.e., acrosspressed-in portions. In the foregoing configuration, therefore, thespacers wound around the noncontact charging roller are pressed againstthe photoreceptor drum at respective positions that are distant by morethan an effective projection length of each of the flanges from therespective opposite ends of the charging roller. The foregoingconfiguration allows precise adjustment of the gap between the chargingroller and the photoreceptor drum, thereby preventing the photoreceptordrum from being nonuniformly charged because of abnormal discharge orinsufficient charging. High-quality image is thus ensured.

The flanges each have an efficient projection length of approximately 5mm as measured from the respective opposite ends of the photoreceptordrum. As shown in FIG. 3, the outside diameter of the photoreceptor drumshows a slight increase at a distance from the opposite ends of morethan approximately 10 mm, i.e., more than twice the efficient projectionlength.

Accordingly, the gap is precisely adjusted by setting the windingpositions of the spacers distant by twice the effective projectionlength to approximately 10 mm from the respective opposite ends of thecharging roller. In order to avoid limitations on a transfer area ?c andan image area ic as shown in FIG. 4(B), it is preferable not to set therespective winding positions of the spacers even more distant from theopposite ends of the charging roller.

The flanges each having an outside diameter smaller than an insidediameter of the photoreceptor drum can be fixedly bonded to therespective opposite ends of the photoreceptor drum with an adhesive thathas a linear expansion coefficient approximately equal to a linearexpansion coefficient of the photoreceptor drum.

In the foregoing configuration, a ultraviolet-curable resin, hereinafterUV-curable resin, having a linear expansion coefficient of 3.0*10⁻⁵ isusable as such adhesive. Since an aluminum base shaft of thephotoreceptor body has a linear expansion coefficient of 2.3*10⁻⁵, thereis a slight difference in linear expansion coefficient and thereforelittle difference in thermal expansion between the UV-curable resin andthe base shaft. Accordingly, little negative effects such as bucklingare caused. Furthermore, the UV-curable resin allows the bondingoperation to be performed with high precision and operability.

Also, negative effect such as buckling is unlikely to be caused when theflanges, the photoreceptor body, and the adhesive to be used for bondingthe flanges to the photoreceptor body have approximately equal linearexpansion coefficients.

Approximately equal linear expansion coefficients of the flanges and thephotoreceptor body also allow the bonding operation to be performed withhigh precision. For example, a combinatoin of the aluminum base shaft ofthe photoreceptor body that has the linear expansion coefficient of2.3*10⁻⁵ and flanges each including an ABS resin that has a linearexpansion coefficient of 3.0*10⁻⁵ (e.g., Asahi Kasei Corporation ProductNo. R420) results in an increase of 3.2 μm in the outside diameter ofthe photoreceptor body at a temperature rise of 30° C. The increase haslittle negative effects.

Since conventional plastic resins have linear expansion coefficients ofapproximately 10*10⁻⁵, it is preferable to selectively use a resinmaterial having a small linear expansion coefficient.

The spacers may be each wound with a single turn around the noncontactcharging roller, with opposite ends of each spacer cut at an angle andarranged so as to face each other across a gap of predetermined width.Alternatively, the spacers may be each wound with a plurality of turnsaround the noncontact charging roller. Also, the spacers may be eachwound with a single turn around the noncontact charging roller, withopposite ends of each spacer cut at an angle and one end overlapped bythe other end on the charging roller. Furthermore, the spacers may eachhave two parts that are shorter than the circumference of the noncontactcharging roller, and the two parts may be wound adjacently around thecharging roller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating configuration of a relevant part of animage forming apparatus according to embodiments of the invention;

FIGS. 2(A) through 2(D) are views illustrating a manner in which aphotoreceptor drum and each of flanges of the image forming apparatusare fitted together;

FIG. 3 is a graph indicating a change in outside diameter of thephotoreceptor drum observed between before and after the flanges arepressed into the photoreceptor drum;

FIGS. 4(A) and 4(B) are diagrams illustrating an arrangement of anoncontact transfer roller and the photoreceptor drum of the imageforming apparatus;

FIG. 5 is a diagram illustrating a manner in which a spacer is woundaround the noncontact charging roller;

FIG. 6 is a diagram illustrating another manner in which the spacer iswound around the noncontact charging roller;

FIG. 7 is a diagram illustrating another manner in which the spacer iswound around the noncontact charging roller;

FIG. 8 is a diagram illustrating another manner in which the spacer iswound around the noncontact charging roller;

FIGS. 9(A) and 9(B) are diagrams illustrating spacers as wound aroundboth the photoreceptor drum and the noncontact charging roller;

FIG. 10 is a graph showing results obtained from simulations ondeformation of a base shaft of the photoreceptor drum having a diameterof 30 mm, with the flanges pressed thereinto;

FIG. 11 is a graph showing results obtained from simulations ondeformation of a base shaft of the photoreceptor drum having a diameterof 40 mm;

FIG. 12 is a graph showing results obtained from simulations ondeformation of a base shaft of the photoreceptor drum having a diameterof 50 mm;

FIG. 13 is a graph showing actual measured deformations of the baseshaft having the diameter of 30 mm with the flanges pressed thereinto;

FIG. 14 is a graph showing normalized values obtained by normalizingsimulated deformations of the base shaft of the diameter of 30 mm withthe flanges pressed thereinto, with respect to a maximum deformation;

FIG. 15 is a graph showing normalized values obtained by normalizingsimulated deformations of the base shaft of the diameter of 40 mm, withrespect to a maximum deformation;

FIG. 16 is a graph showing normalized values obtained by normalizingsimulated deformations of the base shaft of the diameter of 50 mm, withrespect to a maximum deformation;

FIG. 17 is a graph showing normalized values obtained by normalizingsimulated deformations of the base shaft of the diameter of 30 mm, withrespect to a wall thickness of the base shaft;

FIG. 18 is a graph showing normalized values obtained by normalizingsimulated deformation of a base shaft of a wall thickness t of 0.8 mmwith the flanges pressed thereinto, with respect to a diameter D of thebase shaft; and

FIG. 19 is a graph showing normalized values obtained by normalizingsimulated deformations of the base shaft of the diameter of 30 mm, withrespect to a wall thickness of the base shaft.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a view illustrating a configuration of a relevant part of animage forming apparatus according to embodiments of the invention asdescribed below.

The image forming apparatus includes a noncontact charging device 1, acharging roller 1 a, a cleaning mylar sheet 1 b, a photoreceptor drum 2,a two-component developing device 4, a developing roller 4 a, atransferring roller 6, and a charge-regulating/scattering brush 7. Thecharging roller 1 a corresponds to the noncontact charging roller of theinvention. The charging roller 1 a is magnetized, and is biaseddownwards by a spring. The photoreceptor drum 2 is driven to rotateclockwise in FIG. 1. The developing roller 4 a is magnetized and isdriven to rotate clockwise in FIG. 1. A recording medium 5 as shown inthe figure is transported at a predetermined transport speed (e.g., aprocess speed of 130 mm/s). There is a gap 3 of 40 μm betweencircumferential surfaces of the charging roller 1 a and thephotoreceptor drum 2.

The noncontact charging device 1 has two functions of charging thephotoreceptor drum 2 and of cleaning the circumferential surface of thephotoreceptor drum 2. To the noncontact charging device 1, a chargingbias (i.e., a direct-current voltage with an alternating-current voltagesuperposed; −600 Vdc+1.8 KVpp/900 Hz) is applied. The device 1 isrotated in an against direction, i.e., a clockwise direction in thefigures, with a circumference speed ratio of the device 1 to thephotoreceptor drum 2 being 0.5:1. While being rotated, the noncontactcharging device 1 charges a portion 2 a of the circumferential surfaceof the photoreceptor drum 2.

A developing roller 4 a is positioned so that there is a gap ofapproximately 2 mm between the roller 4 a and the photoreceptor drum 2.To the developing roller 4 a, a developing bias is applied. The roller 4a is rotated in the against direction, with a circumference speed ratioof the roller 4 a to the photoreceptor drum 2 being 2.25:1. While beingrotated, the roller 4 a feeds toner particles T, which are carried bycarriers C, onto the photoreceptor drum 2, so that an electrostaticlatent image formed on the circumferential surface of the photoreceptordrum 2 by a not-shown exposure device is developed into a toner image ona portion 2 b.

A transfer bias of +2 kV is applied to a transferring roller 6. Theroller 6 is rotated in a “with” direction (i.e., in a counterclockwisedirection in the figure) at a process speed. While being rotated, theroller 6 presses the recording medium 5 against the photoreceptor drum 2and transports the medium 5, so that the toner image formed on thephotoreceptor drum 2 is transferred to the medium 5. On the surface ofthe photoreceptor drum 2 after the toner image is transferred, there areresidues such as untransferred toner particles T or carriers C, as wellas paper dust P from a surface of the recording medium 5.

With a brush bias of +500 Vdc applied, the charge-regulating/scatteringbrush 7 adjusts charge quantity on the circumferential surface of thephotoreceptor drum 2. The brush 7 scatters an electrostatic latent imageremaining on the circumferential surface of the photoreceptor drum 2.The brush 7 also renders the residual toner particles T, carriers C, andpaper dust P less attracted to the circumferential surface of thephotoreceptor drum 2.

Then, the toner particles T remaining on the surface of thephotoreceptor drum 2 are collected onto the cleaning mylar sheet 1 b byan electric field of the charging roller 1 a. The carries C arecollected onto the mylar sheet 1 b by a magnetic field of the chargingroller 1 a. The toner particles T and carriers C as collected arereturned into a toner bin of the developing device 4. Therefore, theimage forming apparatus is not provided with an additional, separatecleaning device. Note that the toner particles T each have a diameter of8 μm while the carriers C each have a diameter of 60 μm. Thus, thecarriers C, which cannot pass through the gap 3 and are blocked by thedeveloping roller 1 a, are collected together with the toner particles Tcarried thereon.

To ensure that the charging and cleaning functions are properlyperformed, the image forming apparatus according to the embodiments hasthe following construction. Spacers 8 as shown in FIGS. 4 through 8 arewound around the charging roller 1 a at respective winding positionsnear opposite ends of the roller 1 a. The gap 3 between the chargingroller 1 and the photoreceptor drum 2 is precisely adjusted by pressingthe spacers 8 against the photoreceptor drum 2.

In a first embodiment as shown in FIGS. 2(A) through 2(D), flanges 9 arefitted with the photoreceptor body 2A by being pressed into oppositeends of the body 2A.

Each of the flanges 9 includes a circular plate 9 a integrated with aninsertion portion 9 b. The insertion portion 9 b has an effectiveprojection length a of approximately 5 mm. The respective windingpositions of the spacers 8 are more than the length a distant from therespective ends of the roller 1 a. An outside diameter D2 of each of theflanges 9 is slightly larger than an inside diameter D1 of thephotoreceptor body 2A. Thus, an outside diameter D3 of the photoreceptorbody 2A increases across portions of the photoreceptor body 2A intowhich the flanges 9 are pressed, i.e., across pressed-in portions.

As is clear from FIG. 3, the outside diameter D3 shows a maximumincrease at each of the opposite ends of the photoreceptor drum 2. Thediameter D3 shows a marked increase, with distance X from each of theopposite ends ranging within 0 to 10 mm. With the distance X exceeding10 mm, the diameter D3 shows a comparatively slight increase. Morespecifically, increase in the diameter D3 is negligibly small with thedistance X exceeding twice the effective projection length a.

Accordingly, in order to avoid effects of the increase in outsidediameter D3 in the pressed-in portions, the respective winding positionsof the spacers 8 are set, for example as illustrated in FIG. 4(B), twicethe effective projection length a distant, i.e., length Xg distant, fromthe opposite ends of the photoreceptor drum 2. The configuration asdescribed above allows precise adjustment of the gap 3 between thecharging roller 1 a and the photoreceptor drum 2, thereby preventing thephotoreceptor drum 2 from being nonuniformly charged because of abnormaldischarge or insufficient charging. High-quality image is thus ensured.

Since the effective projection length a of each of the flanges 9 ispossibly set at 5 mm or shorter, it is preferable that the respectivewinding positions of the spacers 8 are set twice the effectiveprojection length distant, or approximately 10 mm distant, from theopposite ends of the photoreceptor body 2A. In order to avoidlimitations on a transfer area ?c and an image area ic as shown in FIG.4(B), it is preferable not to set the respective winding positions ofthe spacers 8 more distant from the opposite ends of the body 2A.

In another not-illustrated embodiment, alternatively, the flanges 9 maybe fitted with the photoreceptor body 2A by bonding fittings of theflanges 9 and the body 2A. In the case, it is preferable that each ofthe flanges 9 has an outside diameter smaller than the inside diameterof the body 2A and that an adhesive to be used has a linear expansioncoefficient approximately equal to that of the body 2A.

For example, a ultraviolet-curable resin, hereinafter UV-curable resin,having a linear expansion coefficient of 3.0*10⁻⁵ is usable as suchadhesive. The photoreceptor body 2A includes an aluminum base shafthaving a linear expansion coefficient of 2.3*10⁻⁵. Because of a slightdifference in linear expansion coefficient between each other, theUV-curable resin and the base shaft has little difference in thermalexpansion therebetween, thereby causing little negative effects such asbuckling. Also, the UV-curable resin allows the bonding operation to beperformed with high precision and operability.

In fitting the flanges 9 with the photoreceptor body 2A by bonding, whenthe flanges 9, the body 2A, and the adhesive have approximately equallinear expansion coefficients, little negative effect such as bucklingis caused. More specifically, a combination is used of: the UV-curableresin as the adhesive; the photoreceptor body 2A including the aluminumbase shaft which has a linear expansion coefficient of 2.3*10⁻⁵; and theflanges 9 including an ABS resin which has a linear expansioncoefficient of 3.0*10⁻⁵ (e.g., Asahi Kasei Corporation Product No.R420). The combination results in an increase of 3.2 μm in the outsidediameter D3 at a temperature rise of 30° C. Accordingly, the combinationprevents the outside diameter D3 from being increased to such a level asto have negative effects. Since conventional plastic resins have linearexpansion coefficients of approximately 10*10⁻⁵, it is preferable toselectively use a resin material having a small linear expansioncoefficient.

On the other hand, when the charging roller 1 a is rotated in theclockwise direction as shown in FIG. 1, the spacers 8 are subject tofriction against the photoreceptor drum 2 and have a high tendency tobecome unwound. In order to maintain the gap 3 precisely, therefore, itis required that the spacers 8 be tightly wound around the chargingroller 1 a so as not to become unwound under friction.

As shown in FIG. 5, for example, the spacers 8 are wound with a singleturn around the roller 1 a. Opposite ends of the spacers 8 are cut at anangle and arranged to face each other. Each of the spacers 8 consists ofa tape of resin material. In the configuration, the following inequalityis preferably satisfied at ordinary temperatures of 20 through 25° C.:Tb*cos?>p*(Rc+Tp)−Lt=0.1  (1),where Lt (mm) is natural length of the tape, Tp (mm) is thickness of thetape, Rc (mm) is outside diameter of the charging roller 1 a, Tb (mm) iswidth of the tape, and ? is an angle at which the opposite ends of thetape are cut.

The tape of resin material has a linear expansion coefficient ofapproximately 10*10⁻⁵. The charging roller 1 a has an outside diameterof approximately 11 mm. If a metal shaft of the roller 1 a has a linearexpansion coefficient of 1.1*10⁻⁵, there is a difference in thermalexpansion of approximately 100 μm between circumferential lengths of thetape and the roller 1 a at a temperature rise of 30° C.

Accordingly, a difference in circumferential length, i.e., a gap g, of100 μm or longer at ordinary temperatures is provided, so that thecircumferential length of the tape do not become longer than that of theroller 1 a at the temperature rise. Thus, even though the tape issubject to repeated friction, the tape is prevented from becomingunwound, and thus from coming detached or from having the opposite endsoverlapped. Also, the difference in circumferential length is set to besmaller than Tb*cos?, so that the spacers 8 are seamlessly wound aroundthe charging roller 1 a. Thus, the gap 3 is precisely adjusted.

FIG. 6 illustrates another embodiment in which the spacers 8 are woundwith a plurality of turns around the charging roller 1 a that is beingrotated in a direction of arrow W. The spacers 8 are wound seamlessly,with an edge of a turn overlapped with a subsequent turn. As shown inFIG. 6, the spacers 8 of width B are wound, beginning at an end portionP1 and ending at an end portion P2.

A seam between the turns would cause a problem of carrier and tonerparticles being accumulated on an adhesive sticking out through the seamor in a groove formed in the seam. The accumulated carrier and tonerparticles would gradually develop enough to prevent the gap 3 from beingprecisely adjusted.

To solve the problem, the spacers 8 are wound spirally with an edge of aturn overlapped with a subsequent turn, as described above. Since anedge of a first turn is overlapped with a second turn, the edge isprevented from coming detached because of a potential difference incircumferential speed between the roller 1 a and the photoreceptor drum2.

FIG. 7 illustrates still another embodiment in which the spacers 8 arewound with a single turn around the charging roller 1 a that is beingrotated in a direction of arrow W. The end portion P1 is covered withthe end portion P2 so that a diagonal overlap r is formed.

The diagonal overlap r allows the end portion P1 to be covered with theend portion P2 as exposed, thereby preventing the portion P1 from comingdetached. Also, the diagonal overlap r allows a reduced fluctuation inthe gap 3.

FIG. 8 illustrates yet another embodiment in which the spacers 8 eachconsist of two parts T1 and T2. Each of the two parts T1 and T2 isshorter than the circumference of the charging roller 1 a. A vertical,circumferential cross-section of the charging roller 1 a is a circle,and the parts T1 and T2 each have length corresponding to length of anarc of the circle with a central angle of 200 degrees. The parts T1 andT2 are wound adjacently around the charging roller 1 a, so as to beshifted with respect to each other in an axial direction of the roller 1a. Also, the parts T1 and T2 have respective end portions of width daligned in the axial direction.

Since the parts T1 and T2 are shifted with respect to each other in theaxial direction, the respective end portions of the parts T1 and T2 areprevented from facing each other in a circumferential direction of theroller 1 a. Thus, there is no seam between the parts T1 and T2.Accordingly, accumulation of carrier and toner particles is avoided, sothat the gap 3 is precisely adjusted.

FIGS. 9(A) and 9(B) illustrates yet another embodiment of the invention.In the embodiment, first spacers 18 are wound around the photoreceptordrum 2 so as to be pressed against second spacers 28 that are woundaround the charging roller 1 a.

The second spacers 28 have higher abrasion resistance and higherdurability than the spacers 8 in the first embodiment. Also, the firstspacers 18 each have a circumferential length larger than that of eachof the spacers 8. Accordingly, the first spacers 18 and the secondspacers 28 are less subject to abrasion, thereby allowing a reducedfluctuation in the gap 3. Since the spacers 18 are to be replacedsimultaneously together with the photoreceptor drum 2, the chargingsystem has an increased life and improved reliability. Alternatively,only the spacers 18 may be provided for being wound around thephotoreceptor drum 2, with no spacers wound around the roller 1 a.

Now described below are results obtained from simulations on deformationof the photoreceptor drum 2 with the flanges 9 pressed thereinto. Theobtained results have been proved to correspond well to actualmeasurement values. From the results, a preferable relationship isobtained among wall thickness t (mm) of the base shaft of thephotoreceptor drum 2, distance X (mm) from the opposite ends of thephotoreceptor drum 2 to the respective winding positions of the spacers18, and diameter D (mm) of the photoreceptor drum 2.

Used in the simulations were nine (9) base shafts having suitable sizesfor the photoreceptor drum 2. The shafts have outside diameters of 30mm, 40 mm, and 50 mm, each with wall thicknesses t of 0.8 mm, 1.0 mm,and 1.5 mm. The flanges 9 to be pressed into the ends of each shaft hadan effective projection length of 8 mm. Fit tolerance between the shaftsand the flanges 9 were set to +20 μm, +40 μm, and +60 μm, respectivelyfor the shafts of the diameter of 30 mm, 40 mm, and 50 mm. Under theforgoing conditions, deformations (Y (μm)) of the respective shafts wereanalyzed.

The analysis results for the shafts of the diameters of 30 mm, 40 mm,and 50 mm are as plotted in FIGS. 10 through 12, respectively. FIG. 13illustrates a graph showing actual measured deformations (?Y (μm)) ofthe pressed-in portion of an actual base shaft having a diameter of 30mm and a wall thickness t of 0.8 mm.

Also, FIGS. 14 through 16 illustrate graphs showing respectivenormalized values Yn=Y/Ymax for the base shafts of the diameters of 30mm, 40 mm, and 50 mm, obtained by normalizing the measured deformationsY with respect to a maximum deformation Ymax. As is clear from thefigures, it was confirmed that there are three types of curves Yn, forthe wall thickness t of 0.8 mm, 1.0 mm, and 1.5 mm, respectively.

FIG. 17 illustrates a graph showing normalized values Xd=X/(t)^(1/2)obtained by normalizing the distance X with respect to the wallthickness t of 1.0 mm. FIG. 18 illustrates a graph showing normalizedvalues Xd=X/(D/40)^(1/2) obtained by normalizing the distance X withrespect to the shaft diameter D of 40 mm. As shown in FIGS. 17 and 18,the analysis results as plotted fall on a single curve and correspondwell to actual measurement values as depicted by squares in FIG. 17 andby circles in FIG. 18, respectively.

FIG. 19 illustrates a graph showing normalized valuesXd=X/(t·D/40)^(1/2) obtained by normalizing the distance X with respectto the wall thickness t of 1.0 mm and the shaft diameter D of 40 mm. Theanalysis results also correspond well to the actual measurement values,as depicted by squares in FIG. 19, of an actual base shaft having a wallthickness of 0.8 mm and a diameter of 30 mm. From the foregoing results,conditions can be set as follows.

(1-1) The spacers 18 can be pressed against undeformed positions of thephotoreceptor drum 2, irrespective of the wall thickness t of thephotoreceptor drum 2, when the following inequality is satisfied:X/t ^(1/2)=8   (2),where X (mm) is the distance from the opposite ends of the photoreceptordrum 2 to the respective positions at which the spacers 18 are pressedagainst the photoreceptor drum 2, and t is the wall thickness (mm) ofthe photoreceptor drum 2.

(1-2) More preferably, each of the spacers 18 is pressed against thephotoreceptor drum 2 in a region between a position corresponding to apeak of undershoot of photoreceptor deformation curve and a middleportion of the photoreceptor drum 2. In the foregoing state, thefollowing inequality is satisfied:X/t ^(1/2)=12   (3).

(1-3) Most preferably, each of the spacers 18 is pressed against thephotoreceptor drum 2 in a region between a middle portion of thephotoreceptor drum 2 and a position corresponding to a point convergingto 50% or less of the peak of undershoot of photoreceptor deformationcurve. In the foregoing state, the following inequality is satisfied:X/t ^(1/2)=17.5   (4).

(2-1) The spacers 18 can be pressed against undeformed positions of thephotoreceptor drum 2, irrespective of the diameter D of thephotoreceptor drum 2, when the following inequality is satisfied:X/(D/40)^(1/2)=8   (5),where X (mm) is the distance from the opposite ends of the photoreceptordrum 2 to the respective positions at which the spacers 18 are pressedagainst the photoreceptor drum 2, and D (mm) is the diameter of thephotoreceptor drum 2.

(2-2) More preferably, each of the spacers 18 is pressed against thephotoreceptor drum 2 in a region between a position corresponding to apeak of undershoot of photoreceptor deformation curve and a middleportion of the photoreceptor drum 2. In the foregoing state, thefollowing inequality is satisfied:X/(D/40)^(1/2)=12.5  (6).

(2-3) Most preferably, each of the spacers 18 is pressed against thephotoreceptor drum 2 in a region between a middle portion of thephotoreceptor drum 2 and a position corresponding to a point convergingto 50% or less of the peak of undershoot of photoreceptor deformationcurve. In the foregoing state, the following inequality is satisfied:X/(D/40)^(1/2)=18.5   (7).

(3-1) The spacers 18 can be pressed against undeformed positions of thephotoreceptor drum 2, irrespective of the wall thickness t and thediameter D of the photoreceptor drum 2, when the following inequality issatisfied:X/(t·D/40)^(1/2)=10   (8),where X (mm) is the distance from the opposite ends of the photoreceptordrum 2 and the respective positions at which the spacers 18 are pressedagainst the photoreceptor drum 2, t (mm) is the wall thickness of thephotoreceptor drum 2, and D (mm) is the diameter of the photoreceptordrum 2.

(3-2) More preferably, each of the spacers 18 is pressed against thephotoreceptor drum 2 in a region between a position corresponding to apeak of undershoot of photoreceptor deformation curve and a middleportion of the photoreceptor drum 2. In the foregoing state, thefollowing inequality is satisfied:X/(t·D/40)^(1/2)=16   (9).

(3-3) Most preferably, each of the spacers 18 is pressed against thephotoreceptor drum 2 in a region between a middle portion of thephotoreceptor drum 2 and a position corresponding to a point convergingto 50% or less of the peak of undershoot of photoreceptor deformationcurve. In the foregoing state, the following inequality is satisfied:X/(t·D/40)^(1/2)=23   (10).

According to the present invention, as described above, the windingpositions of spacers wound around the noncontact charging roller aredistant by more than the effective projection length of each of theflanges from the respective ends of the roller 1 a. The configurationallows precise adjustment of the gap between the noncontact chargingroller and the photoreceptor drum, thereby preventing the photoreceptordrum from being nonuniformly charged because of abnormal discharge orinsufficient charging. High-quality image is thus ensured.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1) An image forming apparatus, comprising: a photoreceptor drum that hasflanges pressed into opposite ends thereof; a noncontact charging rollerthat is arranged so as to face the photoreceptor drum but to have nodirect contact with the photoreceptor drum; and spacers for maintaininga gap between the photoreceptor drum and the noncontact charging roller,the spacers being wound around opposite end portions of the noncontactcharging roller, wherein winding positions of the spacers are distant bymore than an effective projection length of each of the flanges from therespective opposite ends of the charging roller. 2) An image formingapparatus according to claim 1, wherein the winding positions aredistant by twice the effective projection length to approximately 10 mmfrom the respective opposite ends of the charging roller. 3) An imageforming apparatus according to claim 1, wherein the flanges each have anoutside diameter smaller than an inside diameter of the photoreceptordrum and are fixedly bonded to the respective opposite ends of thephotoreceptor drum with an adhesive that has a linear expansioncoefficient approximately equal to a linear expansion coefficient of thephotoreceptor drum. 4) An image forming apparatus according to claim 1,wherein the spacers are each wound with a single turn around thenoncontact charging roller, with opposite ends of each spacer cut at anangle and arranged so as to face each other across a gap ofpredetermined width. 5) An image forming apparatus according to claim 1,wherein the spacers are each wound with a plurality of turns around thenoncontact charging roller. 6) An image forming apparatus according toclaim 1, wherein the spacers are wound with a single turn around thenoncontact charging roller, with opposite ends of each spacer cut at anangle and one end overlapped by the other end on the charging roller. 7)An image forming apparatus according to claim 1, wherein the spacerseach have two parts that are shorter than the circumference of thenoncontact charging roller, the two parts being wound adjacently aroundthe charging roller. 8) An image forming apparatus according to claim 1,wherein the follwing inequality is satisfied:X/t ^(1/2)=8, where X (mm) is distance from the opposite ends of thephotoreceptor drum to respective positions at which the spacers arepressed against the photoreceptor drum, and t (mm) is wall thickness ofthe photoreceptor drum. 9) An image forming apparatus according to claim1, wherein the following inequality is satisfied:X/(D/40)^(1/2)=8,where X (mm) is distance from the opposite ends of thephotoreceptor drum to respective positions at which the spacers arepressed against the photoreceptor drum, and D (mm) is diameter of thephotoreceptor drum. 10) An image forming apparatus according to claim 1,wherein the following inequality is satisfied:X/(t·D/40)^(1/2)=10, where X (mm) is distance from the opposite ends ofthe photoreceptor drum to respective positions at which the spacers arepressed against the photoreceptor drum, t (mm) is wall thickness of thephotoreceptor drum, and D (mm) is diameter of the photoreceptor drum.